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Hello guys new guy from Alaska and have many questions but Il start with one for now. What is the max safe rpm limit on a 2007 topkick? When driving she shifts at 3100 but while engine breaking down long grades she will run up to 4k now why will she run up to 4k while decelerating and not accelerating does this sound right?
 

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yup as long as its doing that in tow haul only. thats the allison doin its thing
 

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I think I've posted this before, but that is normal for a Duramax/Allison combo. With the Duramax (and most diesels) the RPM limit under power is actually dictated by the speed at which the flame front can propagate across the combustion chamber. Diesel fuel creates a much slower flame front than gasoline, hence the lower RPM limits under power. However, under deceleration the RPM limit is dictated by the strength of the rotating, reciprocating, and valve train parts. Generally the valve train is the weak link, specifically the valve springs. At some RPM the valve springs will not have enough pressure to keep the valves from continuing to open as the lifter comes over the peak of the cam lobe, close the valves quickly enough or keep the valves from bouncing back off the valve seat when they close (all conditions generally refereed to as "valve float"). Valve float condition leads to the potential for piston to valve interference, valve seat damage, and valve spring fatigue which will only cause the rpm limit at which valve float happens to decrease as the springs become weaker.

As it turns out the limits of the Duramax's rotating, reciprocating and valve train assemblies are much higher than the limit of flame front propagation. Hence, the D'max will stop pulling in the low 3ks but can spin to 4k+ on decell.

-TJ
 
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